The present invention relates to a technique for improving the electrical properties of the adhesive bond formed between a metal surface and an electrically conductive adhesive.
In modern electronic devices, there are many applications in which electrically conductive elements are bonded to one another by means of electrically-conductive adhesives. For example, wireless cards are often bonded to thick metal backers, especially those formed from aluminum and its alloys, using electrically-conductive adhesives.
The formation of metal oxide, hydroxide, and other corrosion products at the interface between a conductive adhesive and the metal bonding surface (adherend) can compromise both the electrical and mechanical stability of such adhesive bonds, and thus the performance and reliability of the packaging structure. This problem is particularly troublesome in humid environments, especially in the case of adhesively-bonded aluminum.
Aluminum surfaces are protected by a naturally formed thin layer of aluminum oxide that provides passivation of the metal at room temperature and moderate relative humidity. Although the native oxide of aluminum is a poor conductor, it is thin enough to allow a reasonably low contact resistance for conductive interconnections and resists further degradation of the electrical interconnection. Exposure to higher temperatures and humidity induces a transformation of the aluminum oxide to aluminum oxyhydroxide (AlOOH) and finally, if the transformation is complete, aluminum hydroxide (Al(OH)3). These changes in surface composition are accompanied by a transformation of the original morphology of the thin oxide layer to a xe2x80x9ccorn flakexe2x80x9d type structure and finally to platelets. These changes are associated with layers that are mechanically weaker, and non-passivating (i.e., offer no protection of the underlying aluminum/aluminum oxide surface from corrosion). Because it is possible to change the thickness of the oxide layer by these transformations, the mechanical integrity of an adhesive/aluminum oxide interface becomes weaker. Such transformations can also lead to substantial increases in interfacial electrical resistance through the bond and ultimately to mechanical separation of the bonded surfaces (adhesive to aluminum).
Roughening of surfaces, for example by sand blasting, chemical etching, or anodization has been commonly practiced to enhance adhesion of aluminum/polymer adhesive systems and to provide structural durability in humid or corrosive environments. However, when the bond must be electrically conductive, treatments that improve adhesion by producing a thick oxide layer, such as phosphoric acid or chromic acid anodization, are not suitable because of the poor electrical properties of the thick oxide layers.
The Forest Products Laboratory (FPL) process, developed in 1950, has been found to be effective in enhancing the structural stability of non-conductive adhesive bonds to aluminum in humid environments, especially when used in conjunction with organic corrosion inhibitors. See J. S. Ahearn, G. D. Davis, T. S. Sun and J. D. Venables, xe2x80x9cAdhesion Aspects of Polymer Coatings,xe2x80x9d edited by K. L. Mittal, Plenum Publishing Corp., 1982 p.281. However, treatments for promoting electrical stability of electrically conductive bonds to aluminum using electrically conductive adhesives are not known.
Accordingly, there is a need for new technology for forming bonds between metal substrates, especially aluminum substrates, and electrically-conductive adhesives which promote and maintain good ohmic contact between the adhesive and the metal to which is bonded, even when the bond is subjected to humid environments over extended periods of time.
In accordance with the present invention, it has been found that the increase in electrical resistance of a metal/adhesive bond when subjected to humid conditions for extended periods of time can be reduced or even eliminated by treating the surface to be bonded with a specific organic chemical preferably organic coupling agents such as organosilanes, organotitanates and organozirconates, prior to application of the electrically-conductive adhesive.
Thus, the present invention provides a new technique for enhancing the electrical properties of the bond between an electrically-conductive adhesive and a metal substrate attached thereto. The technique comprises treating the metal surface to be bonded with an organic material that chemically bonds to the metal surface and subsequently is capable of reacting with the polymer in the adhesive. In addition, the present invention also provides new electrical components containing metal elements bonded to electrically-conductive adhesives, the components containing a layer of an adhesion promoter between the substrate and the adhesive, the adhesion promoter comprising a material capable of bonding to the metal of the substrate and also of capable of reacting with the adhesive.